1. Field of the Invention
The invention relates to a method and apparatus for monitoring the production of plastic blow molded containers. More particularly, the invention contemplates the sensing of two general classes of defects found in Reheat Stretch Blow Molded PET containers. The first class of defect relates to the formation of holes in the feet and/or walls of the plastic blow molded containers during the blow molding process. The second class of defect relates to a pre-existing condition in the injection molded preform particularly in the finish area generated either in the injection process or during handling and loading of the preform on its way to the blow molding operation. These defects are identified by monitoring the sound adjacent a predetermined location along the production line of plastic blow molded containers. More specifically the defects are detected by monitoring the sound generated at the point of or within +/−30 degrees of the application of either the preblow air or the high blow air.
2. Description of the Prior Art
The leak testing of tanks, pressure vessels, and containers is an important manufacturing consideration in many different industries. In some instances, the gas-tight or liquid tight integrity of these components is usually determined by some form of a pressure-decay test. With this technique, the unit under test is injected with air to some specified overpressure, and the pressure is monitored for a specified period of time. If the pressure does not decay below a specified value at the end of the designated time period, the component under test is considered to be leak-free.
Another technique involves drawing a vacuum on the component being tested and then completely surrounding it with helium gas. A detector inside the vacuum system notifies the operator if helium is present in the air being pumped from the component.
Still another method involves the pressurization/immersion technique which consists of pressurizing the component, totally immersing the component in water or some other clear liquid, and observing the point of bubble emergence.
Yet another method utilizes a collimated beam of light which is scanned across the component under test. The test component has been pressurized with a tracer gas that strongly absorbs the light. When the light passes through the gas emerging from the source of the leak, the light energy absorbed by the gas produces an acoustic emission which is detected by a microphone. The resulting signal may be processed either as an alarm or it may be processed in coordination with the beam scanning mechanism to indicate exactly where the leak is located.
Another method involves apparatus adopted to detect the sound issued outwardly by the individual blow-molding dies during the blow-molding process wherein the sound is converted to an electrical signal and is compared with a reference signal or level and the faulty burst container is rejected.